Imaging apparatus which employs the line-scan principle is well known. Typically, such apparatus will consist of a high energy radiation source such as an X-ray source, and for the purpose of exemplification herein further discussion will describe X-ray systems in particular. The beam of the source may be collimated into a curtain, usually referred to as a “curtain beam”, and is then detected by a linear array detector for example comprising a linear photodiode array. Image information is obtained by having the object of interest move linearly for example at right angles with respect to the beam and storing successive scans of X-ray transmission information derived from the linear array from which a complete image frame can be compiled.
If the object being scanned is heterogeneously transmissive of x-ray radiation, and for example consists of or contains multiple smaller objects and/or components of dissimilar materials, it can be possible to build up an image of the object, and in a particular case of the contents or components. The image may then be displayed on a viewing screen. This image can be useful for example in relation to the possible applications outlined above. In particular, it can be useful in determining the contents of a container or the internal structure of an object or body.
Even so, the image generated by such an X-ray apparatus is limited. At best it constitutes a two dimensional shadowgraph of the object being imaged. This can make it difficult to interpret.
European Patent No. 610084 describes a method of creating a “2.5D” solid model picture for viewing. A stereoscopic pair of X-ray images is obtained using two diverging curtain beams derived from an X-ray source. These are separated into conjugate slices and the 2.5D image built up from the resulting slice information.
The resultant image is not strictly a three dimensional image (although it is often so referred to) since it is presented on a two dimensional screen rather than by means of full stereoscopic apparatus. Such a 2.5D representation in fact contains psychological cues to depth such as linear perspective, interposition, shading and shadowing rather than the full physiological depth cue known as binocular parallax or stereoscopy which is required for a full three dimensional image.
The method of EP610084 still provides a user with a final image which can be rotated and looked at from different directions and which can give considerable information as to the relative positioning of different objects or compounds. However it does not give information as to the nature of the items which have been located.
UK Patent Nos. 2329817 and 2360685 are examples of methods and systems which can be used to produce full stereoscopic image pairs. They derive ultimately from principles set out in EP0261984. In particular they are subject to the condition set out at column 4 lines 31 to 48 therein which imposes considerable constraints on detector and source beam geometry. Although stereoscopic imaging can be a relatively powerful technique, exploiting full physiological cues in relation to depth information, and thus offering the potential for a user of the X-ray apparatus to identify objects or components much more readily and clearly, it can be complex in practical operation. To exploit the stereoscopic effect, it is necessary for the observer to receive different images at each eye simultaneously. This will necessitate the use of special apparatus. Moreover, a full stereoscopic technique requires precise control of the image collection process having regard to the conditions identified above. If the stereoscopic pair is to be effective, the respective images must be collected with a parallax that closely approximates to that which would be tolerated by the observer's eyes. For these reasons, full stereoscopic imaging has not gained wide acceptance for scanning machines of this type.
Not only do conventional non-stereoscopic apparatus and methods tend to give limited information in a third dimension, but also the images they produce also give limited information about the material content. In essence, at its simplest, all that is being measured is X-ray transmissivity.
In most practical systems even this is measured indirectly. At its simplest, a typical linear array detector comprises in combination a scintillator material responsive to transmitted X-rays, which is then caused to emit lower frequency radiation, and for example light in or around the visible region, in combination with a semiconductor detector such as a silicon or gallium arsenide based detector which is responsive to this lower frequency radiation. The detector merely collects amplitude information, and does not discriminate spectroscopically.
However, it is known that spectroscopic information from the transmitted X-rays could be used to give additional information about the material content of the objects or components being scanned. This has led to development of dual band detectors, which are capable of separately identifying low and high energy bands from the full spectrum of X-ray emissions. Such a dual energy sensor typically comprises a sandwich pair of semiconductor photodiode arrays or the like, in conjunction with a scintillator configuration that is configured such that the respective detectors detect transmission of low-energy and high- energy X-rays. It is known that the X-ray absorption properties of any material can vary spectroscopically, and that the amount by which the absorption properties vary depends in particular on atomic number. This is exploited by the dual energy detector to differentiate generally between objects having lower and higher atomic number elements predominating.
When exploited as part of a security or material identification system, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category. However, even such a system confers only limited information about composition. The organic/inorganic division is crude and approximate, can readily be confused by objects which are superimposed in the X-ray path, and will give no information concerning the crystalline or polycrystalline nature of an object.
UK Patent Nos. 2329817 and 2360685 incorporate dual-energy transmission detectors. Even so, the compositional information given by the arrangement is still limited, for example in that the low/higher energy duality effect can give only a crude approximation of an organic/inorganic split and cannot itself distinguish polycrystalline materials.
For this reason, the references include additional scatter detectors. X-rays are scattered by the materials they pass through and these scattered signals can contain information that may be used to identify the scattering materials. There is great applicability for these detectors as many of the materials that raise security issues, such as explosives, drugs and semiconductor materials, have a polycrystalline structure and therefore produce good scatter signals. This technique of identifying materials from the scattered signal though possible is not currently commercially used as the extra scatter detectors introduce greater complexity in the system and the scattered beams are weak and so throughput is limited.
The line-scan X-ray technique is widely used in security applications where the detection and differentiation of objects of complex and varied shape and composition is an important feature. A better resolution of the exact shape and location of such objects in three-dimensional space would be a considerable improvement on present techniques, especially if composition could also better be characterised.